1. Field of the Invention
The present invention relates generally to the blow-molding container manufacturing process and more particularly to ejecting blow-molded containers from molds.
2. Related Art
One method of producing hollow plastic containers, such as bottles, is blow-molding. Blow-molding is a process of forming a molten tube of thermoplastic material, then blowing up the tube with compressed air to conform to the interior of a chilled blow mold. The most common methods are extrusion, injection, and injection-stretch blow molding.
The extrusion method is usually used to form containers weighing more than 12 grams, such as containers for food and laundry detergents. The method uses a continuously running extruder with a tuned die head that forms the molten plastic tube. The tube is then pinched between two mold halves. A blow pin or needle is inserted into the tube and compressed air is used to blow up the part to conform to the chilled mold interior. The accumulator-extrusion method is similar, however, the molten plastic material is accumulated in a chamber prior to being forced through a die to form the tube.
Injection blow molding is usually used when accurate wall-thickness and/or a high-quality neck finish is needed, or when a polymer cannot be extruded. The method is a process of injection molding a preform (similar in shape to a test tube), then taking the tempered preform to a blow mold to be filled with compressed air to conform to the interior of the blow mold.
Stretch blow molding is used for polymers that are difficult to blow, such as crystalline and crystallizable polymers, e.g. polypropylene and polyethylene terephthalate. Stretch blow molding can be a single-stage process similar to standard injection blow molding, by adding the element of stretch prior to blow forming. Also, a two-step process is possible, where a preform is made in an injection molding machine, then taken to a reheat-stretch blow molding machine for preform reheating and final blow forming in a blow mold.
After a bottle is blow-molded, the mold is opened to allow the bottle to fall out for further processing. Often, however, part of the bottle sticks to the mold. When the bottle sticks to the mold, several problems can occur. First, some manufacturing systems have a safety stop switch. When the bottle becomes stuck in a mold, the manufacturing switch is triggered shutting down the manufacturing line and causing delays in the manufacturing process. Second, the bottle may eventually fall on its own, or may be un-stuck by a conventional ejection-assist system, but deformation of the bottle can occur as a result of sticking, for example, causing the bottle to bend around the sticking part before ultimately falling. A bent bottle will cause problems further down in the manufacturing process, which relies on properly formed bottles in order to operate efficiently.
This is particularly a problem for dual cavity systems. In a dual cavity system, two containers are molded simultaneously. The two containers are typically joined at the top finish end through a moil to form a “log.” In downstream operations, subsequent to molding, trimming and finishing occurs, where, for example, the moil is trimmed off as scrap. The downstream trimming and finishing requires several steps. For example, the log must be transported to a trimmer. In the trimmer, a cutter cuts the moil from the top of each container. The neck is then finished by, for example, reaming or cleaning of the opening. In addition, the flash, or scrap around the seam of the containers must be trimmed either before or after the individual containers are generated by cutting from the log. In many of these downstream operations, proper orientation of the log is required. Orientation of the log can be performed by automated machinery. However, if the log is bent, for example at the moil, the automated machinery may be unable to handle the bottles, i.e. the bent log does not properly fit in the automated machinery. As a result, the downstream processes are affected. For example, the bent log may become lodged in the trimmer, cutter or cleaner resulting in machine malfunction and system downtime. Alternatively, the container may be improperly cut leading to an increase in waste. In either event, system efficiency is decreased.
One conventional ejection-assist system is a pneumatic ejection system. It operates on dual-cavity container “logs” where the two containers are joined at the neck by a moil in the mold. The bottom of each bottle has a tail-flash extending from it. The conventional ejection-assist system has ejector rods that hold the tail-flash while the mold opens. The ejector rods push the log from the mold and then are retracted by timed air-cylinders. However, because the ejector rods push the log from its ends, if the moil in the middle is stuck to the mold, the force of the push will bend the log at the middle, causing problems further down the manufacturing line.
What is needed then is an improved system and method for ejecting blow-molded containers that overcome the shortcomings of conventional solutions.